A pneumatic circuit for supplying compressed air from an air pressure source to an actuator such as a pneumatic cylinder includes a fluid pressure equipment which has a switching valve for switching a flow path, a pressure control valve for controlling pressure, a filter for removing dust, water and the like from the compressed air, and a pipe arrangement for connecting the above components. Similarly, a hydraulic circuit for supplying hydraulic pressure from a hydraulic pressure source to a hydraulic actuator includes a fluid pressure equipment such as a pressure control valve. A pressure gauge for measuring pressure of fluid is used in such a fluid pressure equipment included in a fluid pressure circuit for driving an actuator which is actuated by air pressure or hydraulic pressure as described above.
There is an analog pressure gauge which displays measured pressure using an indicator needle, and a Bourdon tube type pressure gauge provided with a Bourdon tube is frequently used in this type of pressure gauge. In an analog pressure gauge provided with an indicator needle, a scale based on a measurable range of the pressure gauge is marked as a pressure scale of a scale plate indicated by the indicator needle so that the pressure gauge can be used for various fluid pressure circuits. For example, in a pressure gauge used in a pneumatic circuit, a pressure scale based on a measurable range from a minimum value of 0 Mega Pascal (MPa) to a maximum value of several MPa is marked. However, in many cases, air pressure actually supplied to a pneumatic circuit is within a range smaller than a measurable range of the pressure gauge. For example, even if up to 1.0 MPa can be measured by a pressure gauge, in a pneumatic circuit, the pressure gauge may be used within an operating pressure range between 0.4 MPa and 0.6 MPa. In such a case, it is necessary for an operator to check whether or not air pressure supplied to each pneumatic device included in the pneumatic circuit is within a designated operating pressure range by observing the pressure gauge.
Japanese Patent No. 3717621 and Japanese Patent Application Laid-Open Publication No. 06-52215 disclose a pressure gauge provided with a preset marker indicating a pressure range.
A pressure gauge described in Japanese Patent No. 3717621 includes a front cover covering an indicator needle and formed with a transparent circular plate provided with a scale of measurable range, and an opaque holding member for holding the transparent circular plate. Two preset markers are movably provided to correspond to an upper limit value and a lower limit value of a preset pressure range in a gap between the holding member and the circular plate, and each preset marker is disposed on the rear surface side of the scale and protrudes toward the rear surface of the front cover.
In this way, when the preset markers are disposed on the rear surface side of the transparent circular plate provided with the scale of measurable range, the preset markers are overlapped with the scale. Therefore, to check whether or not the indicator needle is located within a preset pressure range, it is necessary to visually check the preset markers that display the upper limit value and the lower limit value through the scale, so there is a problem that it is difficult to identify the preset pressure range at first glance. Furthermore, to change the preset pressure range, it is necessary to remove the front cover and adjust the positions of the preset markers on the rear surface side of the front cover. Therefore, to check whether the adjusted positions are appropriate or not, it is necessary to reverse the front cover and check the positional relationship between the positions of the preset markers and the scale from the front side, so the change of the preset pressure range is troublesome.
On the other hand, a pressure gauge described in Japanese Patent Application Laid-Open Publication No. 06-52215 includes a scale plate provided with a scale of measurable range and a pair of cover plates rotatably attached along the outer circumferential portion of the scale plate. A display band is formed on the outer circumferential portion of the scale plate, and the preset pressure range is displayed by an area of the display band which is not covered by the cover plates by relatively rotating the pair of cover plates that partially cover the display band.
In this way, the cover plates are disposed on the front side of the scale plate and the cover plates are supported by the scale plate, an excessive load is applied to the scale plate during a rotation operation of the cover plates, and there is a risk that the scale plate is deformed or damaged. Therefore, it is necessary to increase the strength of the scale plate by forming the scale plate using a metal material or unduly firmly fix the scale plate. As a result, the fixing operation of the scale plate becomes complex and the cost of the operation increases.